Typically, a utility obtains electric power from one or more primary power generation sources, such as gas-fired, coal-fired, nuclear and/or hydroelectric power plants, for delivery to customers via a distribution grid. The power supplied by these sources is relatively constant, and can be easily regulated to meet the demands of customers while at the same time conforming to the standards for such power, such as nominal voltage and frequency levels. To supplement the power supplied by these primary sources, it is becoming more common to connect secondary sources of power, such as solar panels and windmills, to the distribution grid. Among other advantages, these secondary forms of energy are renewable, in contrast to gas, coal and nuclear sources, and may also help to reduce the emission of greenhouse gases that adversely affect climactic conditions.
When a secondary source of power is connected to the grid, the utility operator expects its contribution to be within certain levels, so that it can be adequately accounted for, and the outputs of the primary sources adjusted accordingly. Unlike the relatively stable output of primary sources, however, the amount of energy produced by secondary sources can vary over a wide range within relatively short intervals, e.g. measured in seconds. For example, the output of a solar panel varies not only in accordance with the time of day, but also as a result of meteorological events such as the sudden appearance and passing of clouds that block direct sunlight. Likewise, the output of a windmill farm is subject to instantaneous gusts and lulls in the velocity of the wind.
A sudden drop in the output of a secondary source is absorbed by the grid, and needs to be accommodated by increasing the output of one or more of the primary sources. Conversely, a sudden upward spike in the secondary output may exceed the transmission capabilities of the equipment on site, resulting in a loss of generated power until the primary source can be curtailed. These sudden changes limit the effective contribution of secondary power sources within the entire fleet of sources. The greater the number of secondary sources that are utilized, the greater the variation in supplied power, which results in reduced reliability for such sources, and/or the need for fast-response primary generating units. This latter requirement induces additional costs at the primary power plants, such as increased maintenance requirements and additional fuel costs associated with operation at a non-optimal set point.
Large swings in voltage can also exceed the distribution system's response capabilities to normal operation. Traditional power generation equipment often can not respond swiftly enough to sudden changes, and incur extra fuel and maintenance costs when attempting to do so.